1. Field of the Invention
The present invention relates to a process for wet regeneration of granular bulk materials charged with impurities and pollutants. In particular, the present invention relates to a process for wet regeneration of clay-bonded used foundry sand.
2. Description of the Prior Art
Foundry sand is typically used in foundries to make molds for the casting of metal shapes. A typical use of the sand is as core sand. Core sand is the sand employed in a core during molding. Core sand may comprise standard molding-sand mixtures or silica sand, usually with a binder.
A process and a device for wet regeneration of clay-bonded "used" foundry sand is known from AT-PS 387 921. In this process, sand is cleaned by two or more friction scrubbing installations (attrition cells) and then de-sludged in a multiple-stage de-sludging and de-watering arrangement to form regenerated sand. A preliminary de-sludging step for removing fine-grain abraded material is carried out between the individual stages to optimize the attrition process.
The cleaned and de-sludged sand, i.e. regenerated sand, is then dried, cooled and stored temporarily. Then the regenerated sand can be supplied to the foundry or another consumer in this form for reuse.
A disadvantage in this process is that it is difficult to construct a waste water treatment stage to employ with this process technology. Consequently, such construction is cost intensive. The waste water removed, as a filtrate, from the cleaning process for treatment is characterized by a small proportion of solids, a high proportion of fine grains in the solids, and a high buffer capacity.
The high proportion of fine grains in the solids, which include fine-grain carbon, quartz and bentonite and other additions depending upon the sands processed, cannot be separated in a simple and economical manner by the process described in AT-PS 387 921. The solids cause a residual turbidity in the waste water. Accordingly, a more extensive cleaning of the waste water with respect to anticipated pollutants, in particular residual phenol, cannot be carried out with the known process due to the residual turbidity in the waste water. Consequently, circulation of the waste water is subject to substantial restrictions.
It would be desirable to provide a process for regenerating sand or other pollutant-containing granular bulk materials which reduces the cost of such regeneration and avoids producing difficult to treat waste water.